Polar loop transmitters have applications in many fields, such as radio, cellular radio, telecommunications, and the like. The term “polar loop” refers to a polar modulation transmitter architecture that applies closed-loop feedback control to both the phase and amplitude of a transmitted signal by using closed-loop control of the transmitted phase as well as the amplitude modulation. In general, an important issue in polar transmitter architectures is the measurement of amplitude and phase distortion in the transmit path (for example, in the power amplifier). In order to compensate for any amplitude and/or phase distortions, adaptive predistortion compensation can be applied to the modulation signal. However, dynamically compensating for distortions by using adaptive predistortion compensation requires feedback from the transmit signal so as to be able to dynamically measure and compensate for the transmit distortions. Due to the fact that the modulation signal is applied to the modulator in polar coordinates, it can be advantageous to have the feedback signal also in polar coordinates. Therefore, a phase feedback receiver and an amplitude feedback receiver may be used to determine the polar feedback signals for compensating for phase and amplitude distortions, respectively.
With respect to the phase feedback signal determination, a Cartesian feedback receiver may be used to convert the radio frequency (RF) feedback signal down to an analog baseband signal, and then successively convert the analog baseband signal to a digital signal using an analog-to-digital converter (ADC). Afterwards, in the digital domain, a Cartesian-to-Polar conversion can be performed. However, the use of a Cartesian receiver, in addition to requiring conversion to Polar coordinates, can be a cumbersome approach to extracting a phase signal. Another disadvantage of the use of a Cartesian receiver for down-conversion is the typically high current consumption of the receiver due to the high signal quality requirements of the Cartesian receiver. Furthermore, the requirements on the ADC can be significant, as well as the fact that two ADCs are required (i.e., for I & Q paths) to extract phase when using the Cartesian receiver approach to phase extraction.